This invention relates to incoming call signaling facilities for communication systems and particularly to circuitry for controlling single and plural phase incoming call ringing for communication line circuits. The invention further relates to time-shared call answer detection and ring tripping control circuitry for communication lines.
It has long been a commmon practice in the telephone industry to alert a called station to an incoming call by ringing a bell of the telephone instrument. Typically, the ringing occurs in cycles, each of which illustratively comprises an active interval of 1-second duration followed by a 3-second silent period. Such ringing is known in the art as noncoded ringing. The silent and active ringing intervals are slightly different for coded ringing which is widely used on multiparty lines and involves multiburst of active ringing intervals. The cycles for both coded and noncoded ringing are recurrent until the incoming call is answered or the call is abandoned. When the call is answered, ringing is terminated, or tripped, as soon as practicable to preclude a transmission of high level ringing signals through the telephone receiver to the answering party.
A problem in many prior art telephone systems is that the call answer and ring tripping circuitry must be uniquely dedicated and assigned to each called station for the entire duration of an incoming call even though it is only functional for a small percentage of the call holding time. Such circuitry is, in a substantial percentage of commercially operating telephone systems, customarily integrated into a trunk circuit which supplies ringing power to a called line via a switching network. The trunk circuit generally receives the incoming call and controls the entire ringing operation.
Each such trunk circuit is generally equipped with apparatus for activating one of a group of ringing selection switches to select an appropriate code and phase of ringing power for periodically actuating the ringer of the called telephone. The call answer detection and ring tripping circuitry in each trunk circuit is frequently a relay, or equivalent device, which is connected to the called telephone line for the recurrent cycles of silent and active ringing intervals and for sensing D.C. loop current flow over the line resultant from a called party answer. Upon detecting such loop current, the ring trip circuitry is effective to interrupt further ringing of the called station. Thereafter, the call answer and ring trip circuitry remains inactive and out-of-service for the remainder of the call. The inactivity and out-of-service factors obviously result in inefficient uses of call answer and ring trip circuitry.
In an endeavor to overcome the inefficient dedication of call answer detection and ring tripping circuitry, the art progressed in recent years to the extent that such circuitry is sometimes divorced from trunk circuits and segregated into ringing control circuits. Each of the latter circuits is switchable onto call connections to perform the entire ringing control job including the detection of a called party answer and the ring trip control functions. Importantly, after these functions are completed, the control circuit is automatically released from call connections before call conversation commences and is then made available for serving another call. Such an arrangement is disclosed, for example, in L. F. Goeller, Jr., U.S. Pat. No. 3,378,650, issued Apr. 16, 1968.
A disadvantage of the Goeller arrangement is that the common control equipment must establish several distinct connections through a switching network. One such connection is to a ringing control circuit for the ringing job. A subsequent connection is to a reserved trunk circuit for call conversation. Such an approach requires many common control equipment work operations to hunt, test and establish the network connections and undesirably requires separate network channel allocations especially for the ringing job.
The need for the latter work operations and special ringing channel allocations has been reduced by another innovation in the art as disclosed in my U.S. Pat. No. 3,492,437 of Jan. 27, 1970. Advantageously, the latter patent discloses a ringing control circuit which is time shared by a plurality of trunk circuits and which is connectable onto the same network connections as the trunk circuits for performing the ringing job. Such an arrangement enables a call answer detector and ring trip control to be shared among a plurality of trunk circuits during active ringing which is time spaced for each sharing circuit. Each such ringing control circuit is equipped with connector relays which are operated by a switching network controller to connect active ringing power and a shared called station answer detector through a trunk and line link network to a called station.
While my foregoing arrangement has proven technically reliable for commercial applications, it teaches that each ringing control circuit be associated with trunk circuits and that it be controlled by a switching network controller for performing the ringing job over connections through the switching network. In certain present day switching systems, such as time division systems, that teaching does not find practical utility because it is not usually feasible to switch ringing current from trunk circuit appearances in the network through time slot switching elements to called lines.
Moreover, no facilities have been heretofore available for eliminating the requirement in my individual ringing control circuits for circuitry which self-determines the active ringing interval and the disconnection of ringing from a call connection through the network. Such a self-determination is made by a timer arrangement in the ringing control circuits. In addition, a need has heretofore existed for eliminating the time consuming common control equipment operations for interrogating the time shared ringing control circuits to check when active ringing has been terminated either independently by the timer or by a called party answer detector.